A Schottky diode that is fabricated on a sapphire substrate is discussed in Non-Patent Document 1. Two kinds of GaN are grown onto a sapphire substrate having a c-plane. First, an n+ contact layer having a thickness of 1 μm is fabricated onto a low-temperature buffer layer. Then, an undoped GaN layer is grown onto the n+ contact layer. After forming a mesa structure, a Ti/Al ohmic electrode and a Pt/Au electrode are fabricated.    Non-Patent Document 1: Gerard T Dang et al. “High Voltage GaN Schottky Rectifiers,” IEEE Tran. Electron Devices, Vol. 47, No. 4, pp. 692-696.